The exhibit shows how journey-to-work mode shares in Australian capital cities could play out in the future on the assumption that each mode continues to grow at the same rate it did over 1996-2011. (1) (2)

That was the period when public transport shrugged off its long-term decline and grew with renewed vigour. The number of trips to work by public transport grew by 58.5% over the 15 years to 2011, compared to -21.8% in the 15 years prior to 1996.

They grew at almost double the rate of car commutes; in fact commuting by bicycle, taxi & motorcycle, and on foot, all grew much faster than driving over 1996-2011. (3)

Assuming a continuation of the 1996-2011 trends, it can be seen that the mode share of cars in Australia’s capitals would continue falling in the future while those of public transport and cycling would increase.

Let me emphasise that I’m not arguing this scenario is likely. After all, it relies on the assumption that the growth trends of the past 15 years will continue unchanged in the future.

But in the sort of time horizon that infrastructure and strategic planners have to deal with it illustrates the difficulties of achieving really big changes in the way large cities work.

I’m only going to consider projections out to 2051; that’s hard enough but anything beyond that is extremely uncertain e.g. that’s time enough for new technologies like autonomous vehicles to have a big and hard-to-know impact. (4)

Given the assumed rates of growth – and especially the differential between cars and the active modes – it’s surprising how modest the divergence would be in the relative shares of each mode by 2051.

Public transport would lift its share from its current 16.8% to 25.1% by 2051. A solid outcome, but not city-changing. Cycling would grow much faster – from 1.1% to 4.4% – but its impact would still be modest because of its small starting base.

Cars would take a reasonably large hit though. Their mode share would fall from 74.7% to 60.7% by 2051, but they’d still be by far the dominant mode.

It seems that changing the pattern of travel in cities, at least as far as mode share is concerned, is no easy matter.

Achieving a 25.1% mode share for public transport would involve a four-fold increase in the number of passengers that would have to be carried on trains, buses, trams and ferries.

Most of the increase in patronage over 1996-2011 was absorbed by spare capacity and eventually by over-crowding; but growth on the scale envisaged in the future would require massive expansion of public transport capacity and a big increase in operating costs.

And cars would still make huge and growing demands on cities. Notwithstanding the fall in their relative importance, the number of work trips made by car would still increase in absolute terms.

In fact they’d more than double by 2051. And the absolute increase would be considerably larger than the absolute increase in the number of public transport trips.

While these projections shouldn’t be taken literally, the exercise highlights the probability that achieving even relatively modest changes in mode share in the future is an enormous challenge.

Politicians and lobby groups (from both sides of the barricades) can’t continue to advocate infrastructure boondoggles. Available funds will need to be spent on the very best projects. But building infrastructure will only be one part of the solution, possibly a minor one.

I expect that finding ways to use existing infrastructure with greater efficiency will be more important. So will finding “soft” ways to manage the level, timing and geography of demand.

The exhibit aggregates work trips across all capitals, but there are already large differences between cities in the share of work trips made by different modes. I’ll look at individual cities shortly.

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The journey to work data for 1976-2011 is from Mees and Groenhart. The exhibit aggregates work trips from all capital cities except Darwin.

The star performer over 1996-2011 though was cycling; it grew 98.0%. The number of work trips by taxis & motorcycles increased 55.0% and walking grew 52.0%. Work trips by car grew much more slowly (32.1% increase).

I’ve extended the curves beyond 2051 to show how big an effect the differences in growth rates and starting points have e.g. public transport’s mode share maxes out at 36.3% and then starts to decline. But it shouldn’t be treated as a representation of what might happen or even could happen, especially beyond 2051.